1. Technical Field
The present disclosure relates to an optical disc medium in which data is optically recorded, and an optical disc apparatus and an optical disc reproduction method for recording and reproducing the data in and from the optical disc medium.
2. Description of Related Art
Currently, various types of optical disc mediums such as a DVD and a Blu-ray (registered trademark) disc (hereinafter, referred to as a BD) have been used as an information recording medium for storing a video image or data. These optical disc mediums have higher storage reliability than a hard disk drive or a magnetic tape. Therefore, usage of the optical disc medium is expanding from conventional storage of AV (Audio and Video) data such as a video image and a sound to long-term data storage.
However, a capacity of the data that can be stored per volume of the optical disc medium is about one-third of the hard disk drive or magnetic tape. Therefore, from the viewpoint of space efficiency during storage, there is a demand for improving the capacity of the data that can be stored per volume without increasing cost of the optical disc medium, and research and development is actively continued. Recently, among BDs, a BDXL (registered trademark) disc having recording density of about 33.4 GB per layer is released as an optical disc medium having the highest volume recording density.
Such an optical disc medium can store data for at least 50 years, and the optical disc mediums having the storage reliability greater than or equal to 10 times a lifetime of about 5 years of the hard disk drive from the viewpoint of long-term data storage. The data for the long-term storage is transferred from the hard disk drive to the optical disc medium, which allows a balance between the long-term storage reliability and reduction of storage cost to be established. Particularly, compared with the hard disk drive in which the power is consumed during the data storage, in the optical disc medium in which no power is required during the data storage, an amount of carbon dioxide emission can be reduced as a green storage, and therefore data center power consumption that becomes a major issue can be reduced.
However, even in the BDXL disc having the highest recording density among the optical disc mediums, the capacity of the data that can be stored per volume is about one-third of the hard disk drive. Compared with the hard disk drive, a more storage space is required for the optical disc medium during the data storage. Particularly, for the usage in a data center where cost demands related to a storage space are high, there is a demand for improving the recording density per volume of the optical disc medium.
A signal processing technology of increasing linear density of recording data is well known as the technology of improving the recording density per volume of the optical disc medium. A PRML (Partial Response Maximum Likelihood) system is generally used as the signal processing method for increasing the linear density.
The PRML system is a technology in which a Partial Response (PR) and Maximum Likelihood decoding (ML) are combined, and is a system in which a most probable signal series is selected from a reproduction waveform based on occurrence of a well-known intersymbol interference. Therefore, a decoding capability is known to be improved compared with a conventional level determination system. With increasing linear density of the optical disc medium, degradations of the intersymbol interference and an SNR (Signal-Noise Ratio) become further problematic.
A high-order PRML system may be used as the PRML system in order to maintain the reproduction capability. For example, in the case that the BD having a diameter of 12 cm has the recording density of 25 GB per recording layer of the optical disc medium, the reproduction capability can be maintained by adopting a PR(1,2,2,1) ML system. A PR(1,2,2,2,1) ML system is adopted for the BDXL disc having the recording density of 33.4 GB per recording layer.
In the PRML system, binary digital data is decoded from the reproduction signal. In order to perform the digital signal processing, a reproduction clock signal is generated by sampling the reproduction signal in synchronization with the reproduction signal in each channel bit. Generally, information on the reproduction clock signal is included in an edge of a recording mark. The reproduction clock signal synchronized with the reproduction signal is generated by detecting phase information on advance or delay of the edge from a portion corresponding to the edge of the digital reproduction signal sampled in synchronization with the reproduction signal. For the BDXL disc, the proper phase information can hardly be detected only from the edge. However, in the PRML system, there is disclosed a technology of detecting the phase information to generate the stable reproduction clock signal using metrics of a plurality of points since a set of signal series diverge from a certain state until passages of the set of signal series converge (for example, see Unexamined Japanese Patent Publication No. 2009-176405).
For the PR (1,2,2,2,1) ML system, a width of the intersymbol interference spreads back and forth up to 2T. Therefore, using an RLL(1,7) modulation rule in which the minimum pitch of the recording mark and a space becomes 2T, a pattern is restricted, and one piece of phase information can correctly be detected at one edge.
However, in the case that the linear density of the recording layer per layer is increased more than or equal to 50 GB, the intersymbol interference is significantly widened, the plurality of pieces of phase information obtained from the edges of the recording marks overlap with each other, the correct phase information is hardly detected. As a result, the correct reproduction clock signal can hardly be generated, and an error having an influence on a binary determination of the PRML system is increased to degrade a bit error rate. In the worst case, the reproduction signal and the reproduction clock signal are hardly synchronized with each other, and the reproduction can hardly be performed.